Countercurrent pulping of cellulosic materials including regulation of active chemical therein

ABSTRACT

A method is disclosed for the continuous treatment of wood chips in a downflow vertical digester by the alkaline process. Wood chips and black liquor are introduced near the upper end of the digester and travel downwardly therethrough to a sole point at which the black liquor is discharged from the digester. Cooking liquor is introduced at a point between the lower end and black liquor discharge point of the digester and passes upwardly to the discharge point, countercurrent to the flow of wood chips, to cook the chips and leave the digester as back liquor at the discharge point. The cooked wood chips are removed from the lower end of the digester. The active chemical in the cooking liquor is maintained constant. Active chemical is further added near the upper end of the digester and the addition thereof is regulated in response to changes in the residual active chemical in the black liquor which are monitored at the discharge point of the digester, in order to keep the residual active chemical in the discharged black liquor at a substantially constant concentration. The quantity of active chemical added near the upper end of the digester does not exceed 20 percent of the total active chemical added to the digester.

ilite States Patent Slloman [451 Mar, 23 W72 Arthur Raymond Sloman, East Camberwell, Victoria, Australia Associated Pulp and Paper Mills Limited, Melbourne C. I. Victoria, Australia [22] Filed: Sept. 22, 1970 [21] Appl.No.: 74,513

[72] inventor:

[73] Assignee:

lRelated U.S. Application Data [63] Continuation of Ser. No. 648,257, June 23, 1967,

abandoned.

[30] Foreign Application Priority Data June 24, 1966 Australia ..7439 /66 [52] US. Cl ..l62/l9, 162/49, 162/62 [51] llnt.Cl. ....D2 lc 3/26, D21c 7/12 [58] lField oiSearch ..l62/17, 19,49,6l,62, 238

9/1969 Strom et al ..l62l238 X 3/1954 Westcott et al ..l62/62 Primary Examiner--S. Leon Bashore Assistant Examiner-Alfred A. D'Andrea, Jr. Attorney-Pierce, Scheffler & Parker [5 7] ABSTRACT A method is disclosed for the continuous treatment of wood chips in a downflow vertical digester by the alkaline process. Wood chips and black liquor are introduced near the upper end of the digester and travel downwardly therethrough to a sole point at which the black liquor is discharged from the digester. Cooking liquor is introduced at a point between the lower end and black liquor discharge point of the digester and passes upwardly to the discharge point, countercurrent to the flow of wood chips, to cook the chips and leave the digester as back liquor at the discharge point. The cooked wood chips are removed from the lower end of the digester. The active chemical in the cooking liquor is maintained constant. Active chemical is further added near the upper end of the digester and the addition thereof is regulated in response to changes in the residual active chemical in the black liquor which are monitored at the discharge point of the digester, in order to keep the residual active chemical in the discharged black liquor at a substantially constant concentration. The quantity of active chemical added near the upper end of the digester does not exceed 20 percent of the total active chemical added to the digester.

1 Claims, 1 Drawing Figure 15 llllllllllllllllllllliii COUNTERCURRENT PULPING F CELLULOSIC MATERIALS INCLUDING REGULATION OF ACTIVE CHEMICAL THEREIN This is a continuation of application Ser. No. 648,257, filed June 23, 1967 now abandoned,

This invention relates to a method temperatures, which comprises feeding the wood in subdivided form to an entry point of the digester and causing same to move through the digester to a discharge point, the wood being caused to move in succession through a cooking zone and a diffusion-displacement zone in the digester, subjecting the wood to a cooking treatment in the cooking zone by means of an alkaline cooking liquor containing active chemical which is introduced into the digester at a point where the wood is substantially cooked, causing the cooking liquor to pass through the cooking zone in a direction countercurrent to the movement of wood thcrethrough, subjecting the substantially cooked wood to a diffusion-displacement treatment in the diffusion-displacement zone by means of water which is introduced into the digester at a point between the point of entry of the cooking liquor and the point of discharge of the cooked wood from the digester, causing the water to pass through the diffusion-displacement zone and the cooking zone in a direction countercurrent to the movement of wood therethrough, discharging black liquor from the digester at a point between the point of entry of the wood and the point of entry of the cooking liquor, and discharging cooked wood substantially free of active chemical from the digester, characterized in that a further portion of active chemical is introduced at or near the point of entry of the wood into the digester.

The countercurrent digesting and washing of cellulosic materials for the manufacture of cellulosic pulps may be briefly described as follows:

The cellulosic raw material is introduced at one end, for example the top, of a digester vessel, and the chemical in aqueous solution for digesting or cooking the material is introduced at a point between the point of entry of the raw material and the point of discharge of the digested material. Water or other washing fluid is introduced at or near the point of discharge of the digested and washed material. The water or other washing fluid in controlled amount flows in a direction opposite to that of the cellulosic material, and at or near the point of entry of the cooking chemical the chemical and washing fluid are mixed and heated and flow in a direction opposite to that of the cellulosic material. The combined digesting and washing fluid and chemicals and substances dissolved therein leave the digester vessel at a point between the point of entry of the raw material and the point of entry of the cooking chemical.

This process when operated in a steady state with raw materials of substantially constant composition, flows of fluids, dosage of chemicals and temperatures held substantially constant results in the production of cellulosic pulps of substantially constant composition and quality.

It is known in the art that cellulosic raw materials vary in composition and in many instances efforts to compensate for the natural variation in composition are made by mixing raw materials from various locations and of different botanical origin to minimize the variations in the material being introduced to the digesting process. In spite of such measures it is generally not possible to avoid at least long term changes in the raw material and adjustments to the process are therefore necessary from time to time to control the quality of the pulp produced.

In the case of alkaline and neutral processes operated with countercurrent digesting and washing, adjustments are made in either the rate of addition of cooking chemical or the temperature of the digesting liquor to achieve the desired result and such changes are based on tests performed on pulp samples blown from a test cock or the discharge of the digester vessel or from both.

lt will be appreciated by those skilled in the art that in this system a difficulty arises in that changes in the raw material are not detected until such changed raw material has been sampled at the test-cock and pulp has been made therefrom and has been tested, and it will therefore be clear that a change in, for example, the rate of addition of cooking chemical, will be slow to have a correcting effect on the pulp quality. Thus in normal operation at least one hour may elapse before the change is detected and at least one hour more will elapse before a correction can be effective. This hitherto has been a real difficulty, but we have discovered a means for overcoming it; the technique for doing so is the substance of this invention.

In the pulping of cellulosic materials by the alkaline and neutral processes alkali (which term is used in this specification to include caustic soda and other cooking chemicals used in the said processes) is neutralized by three principal means:

1. At the commencement of the digestion by materials readily neutralisable and generally referred to as extractives" consisting of material soluble in hot water and/or dilute alkali.

2. By material referred to in the art as lignin which is neutralized and made soluble only by solutions of alkali of higher concentration than need be used in treating extractives and at higher temperatures than are necessary for extractives.

3. By carbon di-oxide and other degradation products arising from carbohydrate materials and produced under the conditions necessary for removing lignin,

Extractives are neutralized by alkali present in the digesting liquors which have already passed through the section of the process in which lignin has been treated, and to secure maximum utilization of the original chemical introduced to the process the rate of addition of digesting chemical is maintained at such a value that at the point of discharge of the liquors from the vessel the alkali concentration is very low. If the raw material entering the digester changes in composition so that its requirement of alkali or other cooking chemical changes, then the concentration of active chemical in the liquor discharged from the vessel increases or decreases. ln the case of an increase in concentration more chemical has been present during the reactions than'was necessary, and the pulp will tend to be overcooked; in the case where the concentration decreases, less chemical has been present than necessary and the pulp will be undercooked. To correct this position the rate of addition of cooking chemical is decreased or increased, but it will be clear that some time will be required before the system again reaches a steady state, producing pulp of satisfactory quality and with the concentration of residual alkali in the discharged cooking liquor at the required level.

It has been shown that with many raw materials of the kind used in the art and in particular with materials of similar botanical origin, the quantity of chemical neutralized in the reactions involving lignin and carbon di-oxide, that is, those occurring in the so-called delignification section of the cooking reactions, are relatively constant, and that the major change in chemical requirement occurs as the result of changes in the so-called extractive content. While it may seem that preliminary tests of the raw material would reveal such changes and permit anticipatory action to be taken in the digesting process to meet the change, the methods for doing this are slow and laborious and have not been developed to a point where such control measures can be taken. The present invention obviates the heed for such tests and improves the unifomiity of the pulp produced even when marked changes in chemical requirement occur.

The reactions involved in neutralizing extractives occur with relatively low concentration of chemical and at temperatures as low as l30150 C., although they occur much more rapidly at higher temperatures. Such reactions in countercurrent pulping will be near completion in the cellulosic material at a point near to the point of discharge of the spent cooking liquor when the process is operating in a steady state, so that the concentration of chemical in the liquor will be at a low value at this point. Commonly, for example, with eucalyptus woods, some 10 percent NaOl-l/ovendry wood is readily neutralisable in this extractive section. Because of the nature of the extractive they may be neutralized with weaker bases than sodium hydroxide but for the sake of clarity in this specification we refer to caustic soda as the chemical entering these reactions. It will be understood that we are not limited to the use of caustic soda for this purpose.

Because of the speed of the reactions occurring in the neutralization of extractives the concentration of active chemical in the cooking liquor is much greater at a point just prior to the liquor reaching the discharge point for the liquor, and it has been found that the reaction rate in terms of rate of neutralization of alkali in the delignification section of the digestion is much slower and is a function of the concentration of active chemical present during these reactions. Important also is the fact that the materials present in the wood which are involved in the delignification reactions neutralize relatively much smaller amounts of chemical. For example, in the case of eucalyptus woods, such materials neutralize approximately 4 to 5 percent NaOH/o.d. wood. It will be seen therefore, that if, for example, the amount of chemical required by the extractive increases and insufficient chemical is available at or near the point of discharge of the liquors from the vessel to satisfy this demand, then as the cellulosic materials move forward against the cooking liquor the extractives will continue to neutralize chemical and thus the position of the interface at which extractive neutralization is substantially completed will move closer to the point of addition of new chemical, thus decreasing the time available for the delignification reactions to take place. While this is occurring, no change has been made in the rate of addition of cooking chemical so that the gradient of concentration from the point of entry of chemical to the said interface has not changed and the pulp when it reaches the test-cock will be undercooked.

If the amount of chemical required by the extractives decreases, then at the interface where the extractives are neutralized the concentration of active chemical will increase, the said interface will move further away from the point of addition of new chemical, the material will then be subjected to a higher concentration of active chemical at the original position of the interface, and as the wood advances it will require less chemical than previously. Thus the concentration of active chemical at any point in the delignification process will tend to increase and the pulp will be overcooked. In addition to this, the excess chemical will be discharged from the vessel without being neutralized and this is an economic disadvantage.

We have discovered that if the concentration of active alkali in the discharge black liquor is monitored, changes in the chemical requirement of the raw material are revealed by changes in this concentration and, because of the speed of the extractives reactions, immediate remedial action can be taken by the addition of controlled amounts of chemical. These amounts of chemical are preferably relatively small and are preferably added in amounts which are regulated according to the requirements of the raw material as indicated by the monitored changes in the concentration of active chemical in the black liquor being discharged. Such chemical is preferably added to the system at or near the point of entry of the wood into the digester, and is referred to herein as corrective" alkali. This procedure ensures that adequate chemical is available to substantially complete the extractives reaction at or near the point of discharge of black liquor. Thus the process can then be operated so that the cellulosic material, after the extractives neutralization, enters the delignification zone with the extractives substantially neutralized at all times. This prevents any substantial movement of the position of the notional interface between the extractives neutralization zone and the delignification zone, and the rate of the main addition of cooking chemical can be maintained substantially constant because the gradient of concentration of cooking chemical in the delignification zone is now substantially constant and unaffected by changes in the demand for chemical arising from changes in extractive content in the raw material entering the system.

In order to maintain reaction rates at a high value in the delignification zone, not less than percent of total chemical required for pulping the raw material to the desired state is preferably added at the point where delignification is substantially complete, and the rate of addition of chemical at this point is maintained substantially constant for a substantially constant rate of addition of raw material and substantially constant temperature of cooking liquor. Corrective" chemical is then added in amount preferably not greater than 20 percent of the total alkali addition, near the point of discharge of black liquor, either with the raw material at entry or otherwise, in quantity sufficient to ensure that the residual active chemical in the discharge liquor is maintained substantially constant.

One form of the invention will be described with reference to the apparatus illustrated diagrammatically in the accompanying drawing wherein the FIGURE is a sectional view, partly broken away for convenience of illustration, showing a Kamyr type continuous digester which has been modified to enable the process of the present invention to be carried out therein.

The digester comprises a cylindrical pressure vessel 10 mounted with its axis vertical to the upper end of which the raw cellulosic material is added continuously or intermittently. For convenience, wood in the form of chips as usually used in the industry, is employed.

The wood chips are fed to the upper end 10a of the digester 10 as shown at +by means +in the art. Black liquor is also fed to the upper end of the digester 10 as shown at 12, through supply line 12a and corrective alkali in the form of white liquor, which may be preheated, is added through supply line 16a and is mixed with the black liquor at 13. The corrective alkali is added under the control of valve 26, as hereinafter described.

The digester 10 is provided at a point between its upper end and its midpoint with a set of upper internal screens 14 and a discharge outlet 15 for black liquor; and is provided at a point between its midpoint and its lower end with a liquor inlet 16 and with a set of lower internal screens 17. Cooking alkali in the form of white liquor is introduced at or near point 16 and is heated, by means known in the art and not shown in the drawing, to the required temperature (say to C., usually about 180 C.). Heating of the incoming cooking liquor may be effected in various ways indirectly or by the addition of direct steam to the vessel.

A test cock 18 provided at a point below inlet 16. Cooked wood chips are discharged from the lower end of the vessel 10 through outlet 19 by means known in the art (not shown).

An inlet 20 is provided in the lower end of the digester 10 through which a washing fluid, usually water, may be introduced into the digester, said water or other washing fluid being supplied under pressure through a pipe line (not shown) and by means ofa pump (not shown).

The section of the digester marked A in the drawing is referred to as the section in which the neutralization of extractives takes place, and the section of the digester marked B is that section where delignification reactions take place. The horizontal dotted line D represents the notional interface between the zones A and B. Section C of the digester is the section where countercurrent washing of the cooked wood chips occurs.

Water or other washing fluid is caused to flow upwardly in section C countercurrent to the downflow of wood chips and effects removal from the cooked wood chips of cooking chemical contained in the said chips passing downwardly through this zone.

The hot cooking liquor introduced at 16, together with washing fluid and materials dissolved therein during its passage through section C, pass upwardly through the cooking zone B countercurrent to the flow of wood chips through said zone and effect delignification and in zone A effect extractives neutralization. Black liquor is discharged through outlet 15.

Black liquor being discharged through outlet 15 is sampled through a cooled tube 21 and tested for its content of apparent active alkali by means of an automatic titrimeter 22. The rate of addition of corrective alkali at 13 is regulated by means of valve 26 according to the apparent active alkali content of the black liquor discharging at 15 as determined by the titrimeter 22. Upon being discharged from the digester the aqueous pulp may be passed through a concentrator wherein more or less of the contained water is extracted from the pulp. This extracted water is or may be returned to the digester. The degree of extraction of water from the discharged pulp is used to assist in regulating the upflow of water and liquor in the digester.

It is desirable, in effecting the improved procedure of the present invention to regulate the rate of discharge of black liquor from the digester in gallons per minute so as to be equal to or slightly greater than:

U S M W where U is the net amount of water introduced into the digester at the bottom in gallons per minute, S is the rate of introduction of condensed steam with the wood expressed in gallons of water per minute, M is the rate of introduction of moisture in the wood in gallons per minute, and W is the rate of introduction of cooking liquor in gallons per minute.

Advantageously, the rate of upflow of water in the digester is adjusted to not less than lT/Y gallons per minute, and preferably not less than 19T/Y gallons per minute, where T is the output of the digester in lbs. o.d. pulp per minute and Y is the percentage yield of unbleached pulp based on original o.d. wood fed to the digester.

In certain cases it may be advantageous to subject the wood to a prehydrolysis treatment in the digester prior to the cooking treatment. This may be effected by adding water or an aqueous acid solution to the digester at a point above the point of discharge of the black liquor and moving it upwardly countercurrent to the movement of the wood in order to effect prehydrolysis thereof, and withdrawing this water or solution from the digester at a point nearer the point of entry of the wood.

As one example of the invention, a continuous digester as shown in the Figure is operated with the addition at 11 of wood chips derived from eucalyptus at a rate equivalent to 210 lbs. oven-dry wood per minute, the addition at 12 of 30 gallons/minute of black liquor, and the addition at 13 of corrective alkali equivalent to 1 percent NaOH/ovendry wood. In steady rate operation white liquor is added at 16 at a rate equivalent to the addition of 17 percent NaOl-l/o.d. wood, i.e., 35.7 lbs. NaOH/minute at the heating circulation, and the equivalent of 1 percent NaOH/minute is added with the black liquor, i.e., 2.1 lbs. NaOH/minute. Under these conditions the black liquor being discharged from the digester at contains 17 g.p.l. apparent active alkali when determined by potentiometric titration with hydrochloric acid to pH 8.0.

The rate of discharge of liquor at 15 in normal operation is 120 g.p.m. When the residual alkali content in the black liquor discharged at 15 increased to 18 g.p.l., the amount of corrective alkali being added to the black liquor at 13 was reduced by an amount equal to:

(l8l7) l 10/100 lbs./minute 1.2 lbs., so that the total alkali being used was then (35.7+2. ll .2) 36.6 lbs. NaOH/minute. When the apparent active alkali in the black liquor decreased to 16 g.p.l., indicating an increase in extractives present in the wood entering the system, the amount of corrective alkali was increased by an amount equal to:

(l7-16) 120/l00 lb.NaOH/minute l.2 lbs. NaOl-l/minute and the concentration of apparent alkali in the black liquor returned to 17 g.p.l. in each case.

The method of this invention can be carried out by automation, for example in the following manner. The black liquor being discharged at 15 is continuously sampled through a cooled sampling tube 21 so that it issues without flashing. A sample of the stream is taken and by means of an automatic titrimeter 22 the apparent alkali content of the black liquor is determined as frequently as necessary, e.g., every 5 minutes.

The results of the apparent alkali content determinations are transmitted throu connection 21a to a computer memory 23 where the resu ts are stored and by means of a computer 24 (connected to the computer memory 23 by connection 23a) a running average of such determinations is compiled, taking in for example the current and previous live results. A flow meter 25 measures the flow of black liquor from outlet 15 and a signal from this is also transmitted through connection 24a to the computer 24.

The product of the flow and apparent alkali concentration of the black liquor being discharged at 15 is calculated by the computer 24 and by means of a suitable program in the computer 24 the change in the position of valve 26 which is required to maintain the said product at a value preset for the operation, is calculated by the computer 24. The required change in the position of the valve 26 is transmitted through connection 24b to the controller 27 which effects the required operation of the valve 26 through connection 270. A suitable time delay is incorporated in the control circuit to allow for a predetermined delay between the time of changing the valve 26 and the time at which the corrected black liquor reaches the sampling point 21, so that hunting in the system is avoided.

We claim:

1. In a method for the continuous production of pulp from wood by an alkaline process in a substantially vertical digester at superatmospheric pressure and elevated temperature involving the successive steps of feeding the wood in subdivided conditions to the digester at or in close proximity to its upper end and passing the same downwardly through the digester by gravity, effecting delignification of the wood by a cooking treatment in the digester by means of cooking liquor containing active chemical which is introduced into the digester at a point intermediate between its upper and lower ends and is passed upwardly in the digester countercurrent to the flow of the wood therein and is discharged from the digester as black liquor at a sole black liquor discharge point between the upper end of the digester and the point of introduction of the cooking liquor, effecting separation of resulting black liquor from the substantially cooked wood by a diffusion-displacement treatment in the digester by means of water which is introduced into the digester below the point of introduction of the cooking liquor and is passed upwardly in the digester countercurrent to the flow of the pulp therein and is withdrawn from the digester with the black liquor at said discharge point, and discharging cooked pulp substantially free of cooking chemical from the lower end of the digester, the improvement which consists in a. introducing black liquor at or in close proximity to the upper end of the digester, and passing said black liquor concurrently with the wood to said discharge point,

. substantially continuously monitoring the concentration of residual active chemical in the black liquor which is discharged at the said discharge point,

e. introducing a further portion of active chemical at or in close proximity to the point of entry of the wood into the digester,

. the quantity of active chemical in the said further portion referred to in (c) being not greater than 20 percent of the total active chemical added to the digester,

e. automatically regulating the said further quantity of active chemical referred to in (c) in response to changes in the concentration of residual active chemical in the black liquor discharged at the said discharge point so as to maintain the concentration of residual active chemical in the discharged black liquor, as hereinbefore defined, at a low value and substantially constant,

f. maintaining the cooking temperature in the digester substantially constant, and

maintaining substantially constant the rate of feed of active chemical in the cooking liquor, calculated on the rate of feed of the wood on an oven-dry basis. 

